Cloning of a Human Homologue of Mouse Reticulocalbin Reveals Conservation of Structural Domains in the Novel Endoplasmic Reticulum Resident Ca²⁺-Binding Protein with Multiple EF-Hand Motifs

Abstract

Recently, we described the isolation of a mouse cDNA clone encoding a novel Ca²⁺-binding protein, tentatively designated as reticulocalbin [Ozawa, M. and Muramatsu, T. (1993) J. Biol. Chem. 268, 699-705]. Reticulocalbin is a lumenal protein of the endoplasmic reticulum (ER) with a molecular weight of 44, 000 and has six repeats of a domain containing the high affinity EF-hand Ca²⁺-binding motif. The protein has the sequence, His-Asp-Glu-Leu (HDEL), at its carboxy terminus, which serves as a signal for its retention in the ER of cells. To examine the importance of the putative Ca²⁺-binding domains as well as the carboxyterminal HDEL sequence, we have cloned the human homologue of reticulocalbin. The sequence of this clone revealed a novel protein with 95% identity in amino acid sequence to the mouse reticulocalbin, indicating that this molecule has been evolutionarily conserved in mammals. As was found for the mouse reticulocalbin, the human homologue showed six repeats of a domain containing EF-hand motifs. Interestingly, conservation of the amino acid sequence was not restricted to the Ca²⁺-binding motifs, consistent with the possibility that reticulocalbin plays some role(s) besides Ca²⁺-binding. As was found for the mouse homologue, the protein has the HDEL sequence at its carboxy terminus instead of the Lys-Asp-Glu-Leu (KDEL) sequence, which is more common as a signal for the retention of resident proteins in the ER of animal cells. The conservation of the HDEL sequence in reticulocalbin in both species raises the possibility that this sequence has some roles in the function(s) of this protein family.